Add project scaffolding

Makefile, Dockerfile, CI workflow, prettier config, manifests for
Chrome (MV3) and Firefox (MV2), source directory structure, and
minimal test suite. All checks pass.

src/popup/styles/main.css

@@ -0,0 +1,18 @@
+* {
+    margin: 0;
+    padding: 0;
+    box-sizing: border-box;
+}
+
+body {
+    width: 360px;
+    min-height: 600px;
+    font-family:
+        -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
+    background: #1a1a2e;
+    color: #e0e0e0;
+}
+
+#app {
+    padding: 16px;
+}